The present invention relates to an apparatus for heat treatment, such as a continuous annealing furnace for separate hollow bodies, in particular for hollow glass products such as bottles, pots, wide-mouthed jars, crockery, cathode ray tubes for TV, etc.
Heat treatment, particularly for annealing, of hollow glass products is carried out by heating them to the annealing temperature, then progressively cooling them. Such a treatment is conventionally carried out in a tunnel furnace equipped with a conveyor which causes the objects to be treated to pass through it.
The obviously simplest system for obtaining progressive cooling is to cause air to circulate countercurrent to the bodies to be treated, regulating the flow as a function of the temperature existing at a given location in the furnace. In practice it has been found that even if control is exerted on the entry of the air and on its exit, sufficient thermal homogeneity is not obtained.
Another process consists of causing the glass objects to be treated to pass through a tunnel furnace in which circulate, in a substantially vertical direction, currents of gas (or of air) at the desired temperature. In these tunnel furnaces, one or more longitudinal rows of motordriven fans disposed in the upper part and/or the lower part, or even in the sidewall, of the tunnel cause a circulation of gas which is transverse with respect to the direction of motion of the conveyor carrying the objects to be treated.
The gas, thus mixed, is brought to the desired temperature by heating devices and/or cooling devices distributed along the length of the tunnel. Most often, the heat treatment is a programmed heating or cooling requiring a hot gas obtained with burners or electrical resistances. As a general rule, the gas circulates upwards, and the furnace comprises successive zones in each of which the temperature is individually controllable. In certain of these tunnel furnaces, upper and lower fans alternate such that the flow of air is alternately upward and downward. Some of these furnaces are formed with contiguous chambers, each of which communicates with the neighboring one(s) via a hatch formed in the middle portion.
From the point of view of construction as such, the tunnel furnaces of the above kinds are in fact constituted by coaxial ducts. The central duct where the conveyor circulates is that in which the treated bodies pass through. From place to place, the central duct comprises orifices through which the flows of air are admitted and evacuated respectively. The median duct surrounds the central duct, forming an annular space which contains the circulation systems and the air duct systems, as well as the necessary heat exchangers. The outer duct surrounds the two former ones and forms a second annular space occupied by a thermal insulator. The outer duct and the thermal insulator can be the same thing when the thermal insulator is self-supporting (masonry, rammed earth, etc.)
In particular, an example of a tunnel furnace for the heat treatment of glass objects and formed with contiguous chambers is that described in U.S. Pat. No. 4,012,190. This tunnel furnace comprises an enclosure in which the heat treatment takes place. This enclosure is divided into successive zones, through which passes a lattice conveyor carrying the objects and successively passing them through each of the zones. In the upper part of each zone is mounted a fan intended to cause a more or less vertical circulation of gas in the enclosure of the tunnel furnace, and to collect this gas to then send it into a heating and/or cooling circuit.
The obvious disadvantage of a furnace of this type is that the temperature is not distributed in it in the desired manner. In fact, the axial exhausts create a convergent flow of gas having symmetry of revolution, such that the temperature is not steady in a vertical transverse plane, with the consequence that all the objects to be treated and located in the same transverse row perpendicular to the direction of travel do not undergo the same thermal treatment, according as they are in the middle or at the edges of the conveyor belt. In fact, the objects situated at the middle of the conveyor belt pass through vertically ascending currents of gas and thus undergo a uniform heat treatment. The objects situated at the sides of the conveyor, on the other hand, pass through currents of gas which ascend more obliquely the more they are distant from the center of the conveyor belt. The (oblique) currents of gas are caused by the suction of the fan situated at the center of the upper part of each zone of the tunnel furnace, and are more oblique the wider is the tunnel.
The objects situated at the sides of the conveyor consequently do not undergo a uniform heat treatment on the whole of their outer surface, since as they pass through obliquely ascending gas currents, their surfaces do not completely intersect the currents of gas. The result is a poor annealing treatment and consequently scrap losses which are not negligible.
The above disadvantages are not very notable while the width of the tunnel furnace remains relatively small, but since in recent years the tendency has been to construct furnaces of increasingly greater widths in order to increase the treatment capacity, the problem of the uniform transverse distribution of temperature has arisen with greater urgency, since the scrap caused by poor annealing has increased appreciably with the utilization of wider furnaces.